master key

a key that will open a number of different locks, the proper keys of which are not interchangeable.

1570–80 Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
master key
Also called: pass key a key that opens all the locks of a set, the individual keys of which are not interchangeable

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Example sentences
In an individually-keyed padlock system, there is no master key, and all of the
  locks must be opened with their own key.
The contractor shall provide padlocks for each disconnect switch and receptacle
  box with a master key for all locks.
He photocopies it and emails the scan, distributing a master key that can
  access a whole floor.
Using his master key, the officer released the lock.
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